This invention relates to a method for liquid phase epitaxial growth of III-V compound semiconductor materials.
Generally, when making a light emitting diode, a laser diode, a transistor or a similar semiconductor electronic device from a crystal having different kinds of semiconductor layers, it is necessary to minimize stress due to a difference between lattice constants of different kinds of semiconductor layers so as to ensure a stable operation of the electronic device for a long time. For instance, in connection with a GaAs/GaAlAs double hetero-structure laser, it has been reported that the stress arising from a difference between lattice constants of the GaAs layer, which is an active region performing laser oscillation, and each of the GaAlAs layers on both sides of the GaAs layer introduces lattice defects and imposes a limitation on the lifetime of the laser element (Journal of Institute of Electronics and Communication Engineers of Japan, 1975, Vol. 57, No. 7, pp. 835-842). Accordingly, it is of prime importance for enhancement of reliability of the device to minimize the stress resulting from a lattice constant difference between such different kinds of semiconductor layers.
Incidentally, in a case of a compound semiconductor crystal consisting of three or more constitutent elements, the lattice constant can be changed by altering the relative amounts of the constituent elements. By way of example, in a case of growing In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y on an InP substrate, it is possible to grow on the InP substrate a single crystal of In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y with the lattice constant held equal or close to that of the substrate. In this instance, the wavelength of radiation or laser oscillation of the In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y single crystal covers as wide a range as 1.0 to 1.7 .mu.m. For growing such a crystal as In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y with good reproducibility by accurately controlling the lattice constant, the temperature and composition ratio of the melt during growth must be placed under accurate control.
In a case of forming the In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y layer on the InP substrate, it is the practice in the prior art to prepare first a melt for the crystal growth by a method (1) which obtains a saturated melt with raw materials accurately weighed so that the components In, Ga, As and P may have experimental values obtained in advance, or a method (2) which obtains a melt with the raw materials accurately weighed so that after the components In, Ga and As may have experimental values obtained previously, P is saturated therein using PH.sub.3, PCl.sub.3 or InP. Then, the melt thus obtained, after being cooled to a supercooled state, is contacted with a substrate to achieve thereon the crystal growth. The method (1) has a defect of difficulty in obtaining the composition ratio of the melt for defining a proper value of the lattice constant difference. With the method (2), since the composition ratio of P is automatically determined in the course of saturation, a proper composition ratio of the melt can be obtained more easily than in the case of the method (1). However, this method involves a process of causing a chemical reaction of such a substance as PH.sub.3 or PCl.sub.3 at a high temperature to liberate and dissolve P in the solution, or in the case of saturating P using InP, calls for a process of separating the InP from the melt after completion of the saturation of P by the use of a special boat for the crystal growth, or a special crystal growth device.
Further, each of the two methods (1) and (2) has a defect that crystal precipitates may be generated by some causes in the melt before the melt reaches a predetermined supercooled state, making it impossible to control with good reproducibility the composition ratio of the melt at the start of crystal growth on the substrate.